Applications of fiber-reinforced composite materials comprising reinforcing fibers and matrix resins have been spreading in various fields including sports and general industry, particularly aeronautics and space because they enable material designs that best exploit the advantages of reinforcing fibers and matrix resins.
As reinforcing fibers, glass fibers, aramid fibers, carbon fibers, boron fibers and the like are used. As a matrix resin, both a thermosetting resin and a thermoplastic resin can be used, though a thermosetting resin is more frequently used because of its ability of easy impregnation into reinforcing fibers. As a thermosetting resin, a resin composition can be used comprising an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a phenol resin, a bismaleimide resin, a cyanate resin or the like and a curing agent or a curing catalyst.
A fiber-reinforced composite material can be produced by various methods. Among the methods, an RTM (Resin Transfer Molding) method comprising injecting a liquid thermosetting resin (a matrix resin) into a reinforcing fiber base placed in a mold such as a metallic mold (a mold for use in the molding of a fiber-reinforced composite material is referred to as a molding mold, hereinbelow) and then curing the resultant product by heating to produce the fiber-reinforced composite material has been focused as a method having excellent low-cost productivity.
When a fiber-reinforced composite material is produced by an RTM method, the procedure is often carried out by first producing a preform obtained by processing a reinforcing fiber base into a shape that is almost the same as that of a desired product, placing the preform in a molding mold and then injecting a liquid matrix resin into the preform.
As the method of producing the preform, several methods are known, including a method in which a three-dimensional braid is produced from reinforcing fibers and a method in which reinforcing fiber fabrics are laminated and the resultant laminate is stitched. As a method having high general versatility, a method is known in which sheet-like bases such as reinforcing fiber fabrics are laminated with a hot-meltable binder (tackifier) and then the laminate is shaped using a mold for use in molding a preform (also referred to as a “preform mold,” hereinbelow).
In the preform production method in which the preform is shaped with a binder, there is a tendency that subsequent impregnation of the reinforcing fiber bases with the injected liquid matrix resin is inhibited by the binder. Impregnability varies in accordance with the amount of the binder used, and superior impregnation with the matrix resin can be achieved with a smaller amount of binder. However, if the amount of binder is reduced, the bond strength between the sheet-like base layers in the preform is decreased and therefore the shape retainability of the preform becomes insufficient. In those situations, a binder-applied reinforcing fiber base which can have sufficient shape retainability even when the amount of the binder is reduced and can also have excellent liquid matrix resin impregnability is keenly demanded.
In Japanese Patent Laid-open Publication No. 2005-194456, a technique of using, as a binder, a resin composition comprising a thermoplastic resin and an epoxy resin and having excellent adhesion to reinforcing fibers is disclosed. When a preform is produced using the resin composition as a binder, bondability is poor because of the thermoplasticity of the binder, and the shape retainability of the preform becomes insufficient when the amount of the binder is reduced. Furthermore, in the production of the preform, it is needed to closely adhere the fabric bases to each other by increasing the temperature of the preform mold to temporarily melt the binder and then solidify the binder by cooling the preform mold.
In Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 8-509921, on the other hand, a resin composition for use as a binder is disclosed, which is a curing-reactive resin composition comprising a combination of an epoxy resin such as a liquid bisphenol A-type epoxy resin and a catalyst such as ethyltriphenylphosphonium acetate, and which is partially cured by heating to increase the detachment strength of the resultant preform. In that case, however, although it is demonstrated that the detachment strength is improved, the amount of the binder cannot be reduced satisfactorily. Furthermore, in that technique, the binder is in a softened state during heating of the preform. Therefore, it is still needed to cool the preform mold to remove the shape-retained preform from the preform mold.
In addition, as shown in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-524171, a resin composition for use as a binder is disclosed in which thermosettability is imparted to a mixture of a thermosetting resin and a thermoplastic resin. However, although a fiber-reinforced composite material produced by molding the resin composition has greatly improved interlayer toughness, the amount of the binder cannot be reduced satisfactorily. Furthermore, also in that technique, cooling is needed to remove the shape-retained preform.
In Japanese Patent Laid-open Publication No. 2012-251044, a reinforcing fiber base is disclosed in which reinforcing fibers are coated with an emulsion-type binder and therefore the amount of the binder can be reduced compared to when a powdery binder is used. However, the binder used in the reinforcing fiber base also has poor bondability because of the thermoplasticity thereof and therefore there is a limit to the reduction in the amount of the binder.
It could therefore be helpful to improve the disadvantages of the above-mentioned conventional techniques and provide: a resin composition for use as a binder that does not require an increase or decrease in the temperature of a preform mold, enables a reduction in the amount of a binder to be used while allowing a preform to keep the shape retainability at a satisfactory level, has excellent liquid matrix resin impregnability, and can provide a fiber-reinforced composite material having excellent designability; a reinforcing fiber fabric base; a preform; and a fiber-reinforced composite material.